1. Technical Field
This disclosure relates to an image capture apparatus and a control method therefor.
2. Description of the Related Art
Conventionally, image capture apparatuses such as digital video cameras and digital cameras often have so-called contrast-detection type autofocus (AF) as an AF function. Contrast-detection type AF uses the sharpness of a captured image as an AF evaluation value, and focuses on a subject by controlling the position of a focus lens so as to maximize the AF evaluation value. The sharpness of an image used as an AF evaluation value may be, for example, the level of high-frequency components extracted from image signals.
According to contrast-detection type AF, the AF evaluation value is computed based on a captured image and hence could change due to factors other than defocus. For example, in the case where captured images are not still images (captured images are moving images), e.g. when capturing images while panning and when a camera shake occurs, the spatial frequency distribution in the images decreases overall, resulting in a decrease in high-frequency components and the AF evaluation value.
The AF evaluation value is used not only in detection of a focus position, but also in determination of a focus direction and control of an AF operation. Therefore, the influence of a change in the AF evaluation value caused by image blur is versatile. For example, in the case of AF control that has a plurality of inner states and transits the inner state to another inner state in accordance with the AF evaluation value and a change therein, the influence of the change in the AF evaluation value caused by image blur varies with each inner state.
Examples of the inner states of AF control include a minute-driving operation (wobbling) and hill-climbing operation. The minute-driving operation is an operation for obtaining an AF evaluation value by intermittently causing the reciprocating driving of a focus lens within a minute range, and for detecting a direction in which the AF evaluation value increases as a direction of a focus position and identifying the focus position. The hill-climbing operation is an operation for obtaining an AF evaluation value at a plurality of positions while driving the focus lens in the focus direction detected through the minute-driving operation, and for identifying a focus position based on a change in the AF evaluation value.
When image blur has occurred, the minute-driving operation is executed first to identify the direction of the focus position and the focus position. In this case, however, as it is difficult to detect a change in the AF evaluation value compared to the case where the image is stable, it takes time to identify the focus position and the direction thereof. Accordingly, the minute-driving operation is transited to the hill-climbing operation to search for the focus position from a wide range. However, the hill-climbing operation is easily influenced by the change in the AF evaluation value caused by image blur as it identifies the focus position based on an increase and a decrease in the AF evaluation value. Therefore, there are cases where the identified focus position is different from an expected position and captured images are unpleasant to view because the operation of tracking the focus position is unstable.
Meanwhile, many of today's cameras have a camera shake correction function. A camera shake correction function optically or electronically corrects image blur by detecting the amount and direction of image blur based on the output from an angular velocity sensor, such as a vibrating structure gyroscope, and on correlation between images captured in chronological order. Japanese Patent No. 3054603 suggests that information of the detected image blur be used in AF control, or more specifically that AF be not performed while the amount of image blur is large. On the other hand, Japanese Patent No. 4438047 suggests that, in the minute-driving operation whereby AF is performed while operating the focus lens minutely and intermittently, AF be continued after changing a driving condition when the amount of image blur is larger than a predetermined amount.
However, according to the method described in Japanese Patent No. 3054603, while AF is not performed because the amount of image blur is large, tracking cannot be performed in accordance with a change in the distance to a subject.
On the other hand, according to Japanese Patent No. 4438047, although the minute-driving operation is continued under a driving condition that takes into consideration the influence of image blur, the influence of image blur is not taken into consideration in the hill-climbing operation. Therefore, when the operational state of AF is transited from the minute-driving operation to the hill-climbing operation during the occurrence of image blur, the result of AF performed in the hill-climbing operation may be erroneous or unstable due to the influence of image blur as described earlier.